1. Field of the Invention
This invention relates to communication systems, and more particularly to the transmission and reception of digital signals.
2. Background of the Related Art
Communication systems are used to transmit and receive signals over a communication channel. One common form of communication system is a telephone system for transmitting and receiving voice and data signals.
FIG. 1 is a pictorial representation of a telephone communication system 10. Central office 11 sends telephone signals over wires 12 to central office 14. Similarly, central office 14 can send signals to central office 11 over wires 12. Wires 12 may be supported by poles 15, or may be buried. Individual homes and businesses are serviced directly or indirectly from a central office.
The telephone communication system 10 is preferably a digital communication system; i.e., digital signals representing digital data are sent over wires 12. Digital signals typically have greater capacity of information and less noise problems than analog signals. When carrying digital data, these wires 12 are often referred to as "T1" lines. The digital data carried by T1 lines can represent voice and other types of analog data, as well as purely digital information.
The data transmitted over wires 12 is typically organized according to a specific format so that receiving offices 11 and 14 are able to recognize and retrieve the data using the same format. In many digital telephone systems, data is organized into "frames." Frames are blocks of serial information that are transmitted consecutively, where each frame includes the same amount of data (bits). Bits of data in a frame are typically surrounded by framing information called framing bits, which indicate the beginning and/or end of a frame.
FIG. 2 shows an example of digital information that has been organized into a frame 16. In this example, 193 digital signals, or bits, numbered 0-192 are individual bits of digital data that have been transmitted serially over wires 12, each bit typically representing a 1 or 0 value. Bits 0-191 are data bits, representing, for example, voice data transmitted over wires 12. Typically, the voice data is organized into separate channels, 8 bits per channel, where each channel can be an individual telephone communication in, for example, the time division multiplexing (TDM) methodology. Thus, 24 channels can be represented in bits 0-191. Bit number 192 is a framing bit F, which is inserted after every 192 bits of voice data and marks the end of a frame of 24 channels of data.
FIG. 3 is a schematic view of several sequential frames 16 of data of FIG. 2. Each frame is transmitted directly following the preceding frame. Framing bits F1, F2, F3, etc. mark the end of their respective frames and form a "bit stream" of framing bits. Each framing bit can be set to a particular level (0 or 1) so that a number of framing bits examined collectively form a specific pattern. Since a receiver typically includes a framing apparatus programmed to examine incoming bits for a predetermined framing pattern, the receiver can find the framing bits and synchronize itself with the data to distinguish each channel in the frame; i.e. it will have correctly "framed" the data.
In addition to marking the beginning or end of frames in digital data, framing signals or bits can also be used to transmit signalling information. Specific framing bits are retrieved and information is extracted according to a standardized decoding method. Signalling information is typically used for operating procedures and servicing and includes phone receiver status (connection made), battery power level, and other service-type information.
In many telephone systems, a number of successive frames defines a "superframe". A framing apparatus in a receiver is typically programmed to examine signals over one or more superframes to distinguish the framing pattern and frame the data correctly. A commonly-used superframe 18 is 12 successive frames, as shown in FIG. 3.
Several different types of framing formats are used in telephone systems, since different systems may use varying signalling and framing structures. Some of the most widely-used formats include Superframe (SF), Extended Superframe (ESF), T1 Data Multiplexer (T1DM), and SLC-96.
SF format includes a superframe of 12 frames, each frame including 193 bits, totalling 2316 bits. One framing bit is placed at the end of each of the 193-bit frames to supply terminal framing, which identifies frame boundaries. Frames 6 and 12 also include signalling framing bits, which are used to send operating information independently of the voice channel bits (0-191).
ESF format extends SF format from 12 to 24 frames, totalling 4632 bits. A framing bit is positioned every 772 bits. The ESF format includes synchronization bits, error checking bits, and four signal bits.
T1DM format is a 193-bit format with one channel (i.e. sync byte), bits 185 through 192, forming an 8-bit synchronization pattern along with an alarm bit and a remote signalling bit. T1DM format uses framing bits in the SF format.
SLC-96 format is very similar to the SF format, including the framing bit which is time shared to supply terminal framing and signal framing. SLC-96 differs from SF in signal framing, which includes two signal channels and a serial datalink.
FIG. 4 is a schematic representation of a prior art receiving apparatus 20 used to frame incoming digital data and includes a clock and data recovery apparatus 22, a framing apparatus 24, and a terminating apparatus 26. With additional reference to FIG. 1, receiving apparatus 20 will typically be provided at receiving offices 11 and 14 or on a pole 15. Clock and data recovery apparatus 22 receives a signal from T1 line 12 and derives a clock signal 21 therefrom. This can be accomplished, for example, by oversampling, as is well known to those skilled in the art. The data signal 23 output of apparatus 22 retains the information contained in the original signal on T1 line 12. A framing apparatus 24 is coupled to the clock and data outputs of apparatus 22 to "frame" the data according to a predetermined frame format. Framing apparatus 24 is connected to a terminating apparatus 26. Terminating apparatus 26 is typically processing equipment such as a performance monitor, another framing apparatus, a routing mechanism, etc. that requires or further processes a framed data signal.
Framing apparatus 24 typically has three outputs. One output is the data signal 25, which is output at the frequency of a clock signal 27. Clock signal 27 is output by terminating apparatus 26 and is used to compensate for any differences in frequency between clock signal 21 and the clock of terminating apparatus 26. A second output of framing apparatus 24 is a frame synchronization signal 28, which tells the terminating apparatus the location of the framing bits in the data signal 23. A third output is signal line 29, which contains the signalling information extracted from the framing bits. Framing apparatus 24 also typically includes error-checking components that monitor the data signal and indicate when framing errors occur.
A problem with the prior art framing apparatus occurs due to the inflexibility of the apparatus to frame data according to different types of framing formats. A typical framing apparatus can frame data according to only one particular framing format without operator input. However, the framing formats used on a particular telephone system are typically changed to other formats when different telephone systems use the same wires 12. If the framing format is changed, a signal must be sent by an operator to a controller inside the framing apparatus indicating that the framing format has changed. Some systems might require an operator to go out to the location of the framing apparatus and switch formats manually with switches, etc. This can be both inconvenient and time consuming, and allows little flexibility in changing framing formats quickly.
Another problem with prior art framing apparatus is the cost and equipment needed to frame data on multiple T1 lines. Often, two or more T1 lines are grouped together to send more digital information or to send digital information at a faster rate; such line combinations are designated "T2", "T3", etc. The prior art apparatus typically must dedicate a single apparatus to each T1 line, so that several apparatus are required to frame multiple T1 lines. The cost of framing multiple T1 lines thus is increased with the addition of more framing apparatus.
What is needed is an apparatus and method that will quickly, conveniently and automatically frame data on a digital transmission line according to several possible different framing formats. What is further needed is an apparatus that will frame data on multiple T1 lines without requiring substantial additional equipment and cost.